Dear RC: I am grateful to find this site. For the most part, it succeeds in presenting a scientific, non-political tone, with lots of useful information. As long as you maintain this non-political stance, the site can do great good. Science is becoming more and more politicized and is in danger, on certain subjects such as this, of losing all credibility in the way that social and political science have done, generally. As a result, we get The Day After Tomorrow and State of Fear, both highly political (the former hysterical as well)and obviously partisan representations of issues. If you can avoid such phrases as “right-wing” or “left-wing” you can contribute much to this important discussion. Yours, Don Baker

Does anyone know what caused the medieval warming? I would assume it is not anthropogenic.

[Response: In the modeling studies shown in Figure 2 above, the relatively warm temperatures between AD 800-1200 or so are due to a combination of factors including (1) a relative lack of explosive tropical volcanic eruptions (which can a substantial global cooling influence) in comparison with later centuries. and (2) relatively high estimated values of solar irradiance (though solar reconstructions exhibiting very large century-scale variability, such as that used in the “GKSS” simulation shown in Figure 2, have recently been called into question–see this article in Science by Foukal et al, and references therein). -mike]

How do we know that these processes are not responsible for current climatic variation?

[Response: The same simulations referred to above indicate that natural factors such as volcanoes or solar variations are not sufficient to produce the observed 20th century forcing. The observed 20th century warming ins these simulations can only be produced through the addition of anthropogenic forcing to the simulations. -mike]

How sensitive are estimates of historic temparature variation to changes lasting less than 30 years?

[Response: It is very difficult to to diagnose the potential climatic response to a forcing we don’t even know (the climatic response to a comet impact is quite different from that to a volcano!). More directly relevant to your question, however, recent modeling studies suggest that an explosive tropical eruption associated with a relatively moderate radiative forcing (imposing perhaps only a few tenths of a degree C cooling in global mean temperature), may nonetheless be associated with much larger regional temperature responses in areas far remote from the actual eruption. These large regional responses result from changes in atmospheric circulation (basically, the pattern of undulation of the jet stream), that are influenced by the vertical and latitudinal gradient in the associated radiative forcing, and which redistribute warmth over the surface of the earth, but have little impact on the average surface temperature. See for example this article: Shindell, D.T., G.A. Schmidt, M.E. Mann, and G. Faluvegi 2004. Dynamic winter climate response to large tropical volcanic eruptions since 1600. J. Geophys. Res. 109, D05104, doi:10.1029/2003JD004151.” and references therein. -mike]

When Climatologists Attack!!
The climatologists are angry, and are on the warpath against the industry-funded Tech Central Station: RealClimate » Temperature Variations in Past Centuries and the so-called "Hockey Stick": …coined by the former head of NOAA’s Geo…

There seems to be a lot of dancing around Mann’s Hockey Stick. I have a simple question and am looking for just a yes or no answer:

If random numbers are put into Mann’s procedure, does a hockey stick shape result?

Thank you
Steve Hemphill

[Response: Thanks for your question, and the opportunity it provides for clearing up yet further “Hockey Stick” disinformation. First, I’ll give you a two word short answer: Absolutely Not! For a detailed explanation, see the new post “On Yet Another False Claim by McIntyre and McKitrick”.

Some Glaciers are Melting – And Some Are Growing
A quick response to a question from Stu’s blog. Dave Turner asks: Please explain the growing glaciers in 90% of the Antarctic, the increasing glaciers in the Southern Alps (New Zealand) and even on Mt. St. Helens. Worldwide most glaciers…

[Response: This seems to be lacking some context… who says 90% of antarctic glaciers are growing? – William]